A ported acoustical enclosure, also commonly referred to as bass-reflex, vented, or phase-inverted speakers, has an open duct, which includes a sound path that communicates the internal box volume of the enclosure with an external portion of the enclosure, in order to produce stronger and deeper low frequency. The box tuning frequency (Fb) of ported speaker is defined by the stiffness (Sv) 16 of the internal box volume of the enclosure and the air mass (Mo) 14 in the sound path of the port. The ported speaker 10 is illustrated in FIG. 1B and the equation is shown as below, which is equivalent to the simple harmonic motion of a spring system 12 shown in FIG. 1A.
  Fb  =            1              2        ⁢        π              ⁢                  Sv        Mo            
If an extra chamber 24, which has the same volume as the ported speaker, is added to the ported speaker, and the port is placed within a partitioning wall 26 between both chambers, the equivalent spring system 22 is shown in FIG. 2A, the box tuning frequency will increase to 1.414 times of single chamber ported speaker. The equivalent simple harmonic motion of a spring system 22 of the two chamber ported speaker 20 is represented by the equation:
      Fb    ′    =                    1                  2          ⁢          π                    ⁢                                    2            ⁢            Sv                    Mo                      ≅          1.414      ⁢                          ⁢      Fb      
If a port 34 is placed within the front wall 36 of the extra chamber the resulting two chamber ported speaker 30 is shown in FIG. 3B. The equation of the equivalent spring system 32 shown in FIG. 3A has two tuning frequencies:
            Sv      ′        =          Sv      2                  F      OL        =                            1                      2            ⁢            π                          ⁢                                            Sv              ·                              Sv                ′                                                                    (                                  Sv                  +                                      Sv                    ′                                                  )                            ⁢              Mo                                          =                                                  1                              3                                      ·                          1                              2                ⁢                π                                              ⁢                                    Sv              Mo                                      ≅                  0.577          ⁢                                          ⁢          Fb                                F      OH        =                            1                      2            ⁢            π                          ⁢                                            Sv              +                              2                ⁢                Sv                                      Mo                              ≅              1.732        ⁢                                  ⁢        Fb            
The lower box tuning frequency of the dual chamber 30 is reduced to 0.577 times of single chamber ported speaker. The upper box tuning frequency of the dual chamber 30 is 1.732 times of single chamfer ported speaker. The two tuning frequencies can be adjusted by changing the port length (L), port cross section area (a) and volume of chamber (Vb). The related equation is:
      Fb    =                  ca        2            ⁢                        1                                    V              b                        ⁢            π            ⁢                                                  ⁢            L                                ,where c is speed or sound.
Additionally, conventional three chamber ported speakers are generally able to provide a lower frequency response than single or two chamber ported speakers, and reduce air turbulence noise, reduce excursion of driver and increase the power handling of driver. A typical three chamber ported speaker 40 is shown in FIG. 4B and consists of two drivers 14,44 mounted on partition walls between the chambers and in separate chambers 16,46, respectively, and sharing a common center chamber 24 having a port 34 which is producing low frequency. The equivalent spring system 42 is shown in FIG. 4A. However, even with conventional three chamber ported speakers the acoustical performance is also hampered by limited low frequency response and air turbulence noise.
There is thus a need to alleviate the problems associated with conventional ported stereo speakers. There is a need to provide a compact stereo speaker to enhance the acoustical performance and extend the low frequency response of stereo speakers and provide a stereo speaker with reduced excursion and increased power handling of a driver within the stereo speaker.